The present invention relates to the method for manufacturing biodegradable molded products. More particularly, the present invention relates to having a matrix based on starch, or materials with similar rheological properties, foamed for use in molded products. In particular, the method involves two or more distinctive steps during microwave processing, giving a product with improved packaging properties including resilience, compressibility and shock absorption.
The area of starch based biodegradable foamed materials is widely discussed in the prior art. In particular, U.S. Pat. No. 6,168,857 has a detailed discussion that may be referenced in relation to this patent.
Containers and Packaging
Articles such as sheets, films and packaging molds made from materials such as paper, paperboard, plastic, polystyrene, and even metals are used in enormous quantity. This can take the form of printed materials, mats, containers, separators, dividers, envelopes, lids, tops, cans, and other packaging materials. Advanced processing and packaging techniques presently allow an enormous variety of liquid and solid goods to be stored, packaged, or shipped while being protected from harmful elements.
Containers and other packaging materials protect goods from environmental influences and distribution damage, particularly from chemical and physical influences. Packaging helps protect an enormous variety of goods from gases, moisture, light, micro-organisms, vermin, physical shock, crushing forces, vibration, leaking, or spilling.
For the purposes of the discussion, many prior art products and processes are seen as not being environmentally friendly. Wherein for the purposes of the present invention, “environmentally friendly” may be characterized as:                Being produced from substantially naturally occurring and renewable, raw materials;        Manufactured in such a way as to cause minimal deterioration to the environment for example via low energy processing and low emission methods;        Producing a product that is biodegradable and not harmful to the environment; and        Production whereby the whole process is sustainable.However it is not intended that this definition be seen as limiting.The Impact of Traditional Materials        
Recently there has been a debate as to which of these materials (e.g., paper, paperboard, plastic, polystyrene, glass, or metal) is most damaging to the environment. Paper, paperboard, plastic, polystyrene, glass, and metal materials each have unique environmental issues that do not meet the definition of “environmentally friendly”. These issues can relate to the biodegradability of the material itself or the method of production, for example, high energy use, damaging by-products and emissions.
Another problem with paper, paperboard, polystyrene, and plastic is that each of these requires relatively expensive organic starting materials, some of which are non-renewable, such as the use of petroleum in the manufacture of polystyrene and plastic. Although trees used in making paper and paperboard are renewable in the strict sense of the word, their large land requirements and rapid depletion in certain areas of the world undermines this notion. Hence, the use of huge amounts of essentially non-renewable starting materials in making sheets and articles therefrom cannot be sustained and is not wise from a long-term perspective.
Starch Based Foams
Recent uses of starches and starch derivatives as the binding agent or sole constituent within molded articles are known. U.S. Pat. No. 5,095,054 is the parent document for this style of product. The parent patent, and the patents citing this patent, recognize the fact that starch can be foamed and molded by means of forming what is known in the art as “destructurized starch”. In the manufacture of destructurized starch, native starch or starch derivatives are mixed with a wide variety of additives such as plasticizers, and heated, solidified and cooled, typically into a mold.
EP-707034 and WO95/07693 both use conventional thermal conductive heating processes that do not lend themselves to the production of thick-walled moldings. Non-homogenous heating occurs when the heating process is reliant on heat conduction as it is difficult to heat the core of the material to the same extent as the exterior. This results in non-uniform foam properties, which is undesirable in protective packaging used for cushioning applications.
A further example includes U.S. Pat. No. 6,168,857 in which the process is only usable in thin walled applications. Thin walled articles are of limited use in terms of protective packaging used for cushioning applications. Thick walled articles are needed where shock absorption properties are required. The method of fashioning articles from sheets used in U.S. Pat. No. 6,168,857 does not allow for the forming of thick sheets.
Another U.S. Pat. No. 5,730,824 utilizes extrusion to produce foam panels. These panels are then laminated together to form thick sheets, which can be wire cut to varying size shapes. There are limitations in this process due to the expensive capital equipment required for manufacturing. As a result of the expensive equipment, the method necessitates shipping ‘air’ as the product can only be made in central locations. In addition the shapes are either very limited or costly because they have to be cut out of sheets instead of molded during the foaming process.
Another example, U.S. Pat. No. 5,801,207, relates to taking foamed starch pieces, placing them in a bag or within layers of sheeting and molding the pre-expanded peanuts into solid foam-in-place molds. The limitations are that the foamed peanuts used to make the molds are very bulky and take up a lot of store space, and again increase expense through having to ship air to the point of use instead of sending dense pellets that can be foamed at point of use. The method is also a complicated procedure for the end-user, as they have to fill and seal bags of foamed peanuts and then mold the bag to the product shape.
Two further patents, WO 9/851,466 and U.S. Pat. No. 5,639,518, utilize dielectric heating in processing the starch based materials.
In WO 9,851,466, the dielectric heating proceeds in one step and does not take into account the changing dielectric properties of the material as it heats, nor the relationship between the rheological properties (for example elasticity and viscosity) and the rate of heating. This results in the material not being heated as rapidly and intensely, thus lowering the potential foaming and product resilience.
U.S. Pat. No. 5,639,518 again does not utilize different steps during processing to take account of the changing dielectric rheological properties of the material as it heats up. Two stages are outlined relating to changes in the microwave frequency from low frequency and then high frequency but not with any reference to varying material properties and a rate of heating profile. This frequency change results in a significant increase in processing expense due to more specialized equipment being required.
In addition, the methods described above often produce foams with varying consistency depending on the shape required and often without the combination of uniform physical and mechanical properties. These properties include density, compressibility, resilience and shock absorption. All of these properties limit the product applications.
A further method for molding starch-based mixtures into articles involves batch-molding an aqueous starch mixture between heated dies. The starch binder is preferably initially in an unmodified, un-gelatinized state within the moldable aqueous mixture. The starch/water mixtures are heated between the molds to a temperature great enough to gelatinize the starch as well as to remove the majority of the water from the moldable mixture. The resulting molded articles can be de-molded, but are initially very brittle until they have been “conditioned” by placing them in a high humidity chamber for extended periods of time in order to reabsorb moisture. While the foregoing batch molding process may have some utility, it does not allow for continuous manufacturing and as such is expensive to run.
Based on the above, there is a perceived difficulty in finding improved methods for manufacturing low cost and environmentally friendly products, which have properties similar to paper, paperboard, polystyrene, or plastic, but are biodegradable and resilient.
An object of the present invention is the provision of the method to produce a foamed product with uniform physical and mechanical properties such as density, compressibility, resilience and shock absorption.
A further object is the provision of a method and product, which overcome some, or all of the above described disadvantages of existing biodegradable foamed products.
Another object is the production of a biodegradable foamed product and method, which provides the public with a useful alternative to existing methods and products.
Further aspects of the present invention will become apparent from the ensuing description which is given by way of example only.